Chap. 34 AMPHIBIANS 703 



are exposed to air. Now and then, the frog seems to swallow — a sign of lung- 

 breathing. Actually it pulls the floor of its mouth downward creating a partial 

 vacuum and air comes into this through the open nostrils. The flaps over the 

 nostrils are then pulled down, the floor of the mouth lifted, the glottis opens 

 and the air escapes the pressure by going into the lungs. At the same time, an 

 exchange of gases has been going on between air and blood, through the lining 

 of the mouth. As it exhales, the frog contracts its abdominal wall and squeezes 

 the lungs. The glottis is pushed open; the flaps over the nostrils are lifted; and 

 the air escapes. Usually, the skin is moist enough for an exchange of gases. 

 Experiments have indicated that more carbon dioxide is given off by the skin 

 than by the lungs. 



Voice. Frogs and toads may have been the first animals to use vocal cords. 

 The sound of their spring choruses still seems to come from ancient marshes. 

 The vocal cords are two folds of the lining of the larynx, below and parallel 

 to the glottis. When a frog croaks, it keeps its mouth and nostrils tightly closed 

 and squeezes air back and forth between the lungs and mouth. During this 

 performance air escapes through slits in the floor of the mouth into the air sacs 

 and dilates them into balloon-like resonating organs (Fig. 34.1). 



Excretion. Along with essential products, metabolism produces harmful 

 ones, usually accompanied with water. The waste products may be gases, 

 solids, or liquids. Carbon dioxide, from the oxidation of carbohydrate and 

 fatty foods, is eliminated through the lungs of frogs, the gills of the tadpoles, 

 and the skins of both. The undigested residue of food is not a metabolic 

 product, except as it contains bile excreted by the liver. 



It is important to any animal that a standard amount of water be maintained 

 in its body, especially so in frogs. The skin, urinary bladder, and kidneys 

 maintain this. Frogs constantly absorb water from the air and soil, as well as 

 from the ponds. A relatively large amount passes into the lymph, blood, and 

 other tissues, and from the kidneys into the urinary bladder. The latter is 

 actually a water reservoir. 



Like those of all vertebrates, the kidneys of the frog are composed of micro- 

 scopic tubules bound together by connective tissue, supplied with nerves and 

 closely associated with the blood (Fig. 34,16). The ureter of each kidney lies 

 along its outer edge and receives the urine from minute collecting tubes which 

 cross the dorsal side of it. These collecting tubes in turn receive urine from the 

 kidney tubules which have completed it from urea brought by the blood from 

 the liver. 



Endocrine Glands. The frog's body is under the elaborate chemical con- 

 trol of the endocrine glands that produce secretions which pass directly into 

 the blood. Some of these influence another gland or structure; others affect the 

 whole organism, its behavior, rate of growth, and symmetry. The endocrine 

 glands of vertebrates are discussed in Chapter 15. 



